The present invention refers to a circuit for reducing the noise on a circuit that may come from voltage references, in particular in dc-dc converters.
It is known that in some types of circuits, such as in switching regulators, the reference voltages are subject to noise due to switching of power devices.
For a better comprehension of this problem, consider for example the case of a known dc-dc converter of the step-down type usable as voltage regulator, as illustrated in FIG. 1. The dc-dc converter 1 has an input terminal 2, set, during use, at an input voltage VIN, and an output terminal 4 supplying an output voltage VOUT lower than the input voltage VIN.
The dc-dc converter 1 further comprises a switch 6, typically a power transistor of bipolar or P-channel or N-channel MOSFET type, the opening and closing whereof is controlled by a drive circuit 10. In particular, the switch 6 has a first terminal connected to the input terminal 2 of the dc-dc converter 1 and a second terminal connected, via a diode 12, to ground.
The dc-dc converter 1 further comprises an inductor 16 connected between the second terminal of the switch 6 and the output terminal 4; a capacitor 18 connected between the output terminal 4 and ground; and a voltage divider 20, comprising two resistors 22, 24 and connected between the output terminal 4 and ground; the voltage divider 20 has a tap 26 supplying a divided voltage VFB which is proportional, through the dividing ratio, to the output voltage VOUT supplied by the dc-dc converter 1.
The dc-dc converter 1 further comprises a differential voltage error amplifier (VEA) 28, which supplies at the output an intermediate voltage VM as a function of the difference between the divided voltage VFB and a reference voltage VREF.
The intermediate voltage VM and a comparison voltage VC supplied by an oscillator 32 and having a saw-tooth waveform with preset frequency are supplied at input to a differential comparator 30 of a pulse-width modulator (PWM) type, which, in turn, generates at the output a control signal VP supplied at input to the drive circuit 10 of the switch 6.
The PWM comparator 30 acts substantially as a Pulse-Width Modulator and outputs a voltage having a square waveform the duty-cycle of which is a function of the voltage supplied by the voltage-error amplifier 28, and the frequency of which depends on the frequency of the comparison voltage VC supplied by oscillator 32.
The dc-dc converter 1 has the problem that switching-on and switching-off of the power switch 6 may cause noise, for example on the reference voltage VREF and on the supply voltage. This noise may even be considerable and may jeopardize the precision and proper operation of the circuit, for example causing undesirable switching of the components, in particular in the case of multiple switches. It is known, in fact, that in individual switches the noise may interfere with the leading and trailing edges of the control voltage VP at the output of the PWM comparator 30, and, in addition, the leading edges of the control voltage VP itself are synchronous with the clock signal CK supplied to the oscillator 32. Consequently, for any switch, at each clock cycle it is possible to temporarily block the output of the PWM comparator 30 (by means of a monostable circuit) for a time sufficient to allow the noise causing transient phenomena to settle to a steady-state condition. In addition, the logic devices for circuit control store the trailing edge of the voltage signal at the output of the PWM comparator 30 and block the value of the control voltage VP until the next clock cycle. In this way, erroneous switchings are prevented, even though the problem of having noisy voltage references is not eliminated.
In the case of multiple switches, which comprise a number of PWM comparators, it is possible to mask only the leading edges, which are synchronous with the clock signal, of the signals coming from the comparators themselves. The trailing edges, instead, are not temporarily correlated with one another, and hence it may occur that a disturbance due to the correct switching of a comparator causes undesirable switching of other comparators.
Similar problems may arise also in other types of devices where, in general, switching of power elements causes noise on the used reference lines.
The aim of the present invention is to provide a circuit for protecting an operating circuit from noise on a voltage supply line.
According to the present invention, an electronic device is provided, comprising a voltage generating circuit outputting a reference voltage, at least one operating circuit having an input terminal connected to said voltage generating circuit, and a noise protection circuit that detects events linked to noise conditions. The noise protection circuit having a switch connected between said input terminal and said voltage generating circuit and having a control terminal and voltage memory circuit connected to said input terminal, said control terminal receiving a control signal activated by said noise conditions to cause alternately opening and closing of the switch.